We have recently observed that the genetic linkage of allotype and heavy chain idiotype in inbred mice breaks down when the second strain involved in a breeding study is one of four strains earlier shown by P.D. Gottlieb to carry an unusual light chain marker. All previous investigations have indicated an absence of a role for light chains in the production of the idiotypes under investigation. We now plan to determine whether this observation applies as well to another idiotypic system, namely, that associated with antistreptococcal antibodies of A/J mice. Another goal is to ascertain whether the heavy chain produced by mice carrying the A/J allotype, but lacking A/J light chains, are capable of interacting with appropriate light chains to express the characteristic idiotype. This will be determined by in vitro recombination studies. In a second line of investigation we will attempt to isolate anti-idiotypc antibodies that are specific for only the heavy chain region. Use will be made of the antibodies produced by those mice which have genes controlling the appropriate heavy chains but not the necessary light chains. Another series of experiments will be designed to selectively stimulate T cells carrying this idiotype. If we can induce the stimulation of the appropriate T cells, it may be possible to answer the question as to whether the idiotype on a T cell receptor is totally independent of a contribution by the light chain.